Resistance variable memory elements, which include Programmable Conductive Random Access Memory (PCRAM) elements, have been investigated for suitability as semi-volatile and non-volatile random access memory devices. An exemplary PCRAM device is disclosed in U.S. Pat. No. 6,348,365 to Moore and Gilton.
In a PCRAM device, a conductive material, e.g., silver or other conductive ion, is incorporated into a chalcogenide glass. The resistance of the chalcogenide glass can be programmed to stable higher resistance and lower resistance states based on a voltage controlled movement of the conductive material within or into and out of the chalcogenide glass. An unprogrammed PCRAM device is normally in a higher resistance state. A write operation programs the PCRAM device to a lower resistance state by applying a voltage potential across the chalcogenide glass and forming a conduction channel. The PCRAM device may then be read by applying a voltage pulse of a lesser magnitude than required to program it; the resistance across the memory device is then sensed as higher or lower to define binary logic states.
The programmed lower resistance state of a PCRAM device can remain intact for an indefinite period, typically ranging from hours to weeks, after the voltage potentials are removed; however, some refreshing may be useful. The PCRAM device can be returned to its higher resistance state by applying a reverse voltage potential of about the same order of magnitude as used to write the device to the lower resistance state. Again, the higher resistance state is maintained in a semi- or non-volatile manner once the voltage potential is removed. In this way, such a device can function as a variable resistance memory having at least two resistance states, which can define two respective logic states, i.e., at least a bit of data.
A typical resistance variable cell 100 is shown in FIG. 1. The chalcogenide glass layer 7 is formed between top and bottom electrodes 2, 4 respectively. There may also be a metal containing layer 5, e.g., a silver layer, between the chalcogenide glass layer 7 and the top electrode 2. The metal layer 5 provides metal ions for the switching operations, and the electrode 2 may also provide metal ions for switching. In the conventional cell 100, the bottom electrode 4 may be formed as a plug within a dielectric layer 3. Typically, the electrode 4 is formed by chemical vapor deposition (CVD) processes. The conventional electrode 4 has some disadvantages. CVD processes result in seams or gaps between the electrode and adjacent structures. Additionally, the CVD processes produce electrodes with rough surfaces. Also, the plug electrode 4 has a relatively large surface area. These disadvantages can diminish the consistency and controllability of a device containing the conventional cell 100.
Therefore, it is desired to have an improved electrode for use in a resistance variable device and a method for forming the same.